A nurse who worked at St. Cloud Hospital for 17 years has been charged with stealing a powerful narcotic from at least 23 patients for his own use and replacing the painkiller with salt water.

Blake D. Zenner, 42, of Kimball, Minn., was charged Tuesday in federal court in Minneapolis with taking hydromorphone hydrochloride, a drug commonly sold as Dilaudid, which is used to relieve moderate to severe pain.

Along with being denied their prescribed painkiller, 23 patients in the same hospital unit developed infections because "there was contamination when the nurse removed the drugs from the IV [intravenous] bag," hospital spokeswoman Jeanine Nistler said Wednesday.

She said that the patients suffered unexplained fever but no long-term consequences from what the hospital termed "unusual bloodstream bacterial infections" or from the shortage of the painkiller.

All of the victimized patients "were contacted and given the opportunity face to face [with hospital officials] to have any concerns addressed," she said. In the wake of the thefts, Nistler said, changes were made immediately to make it "more difficult for things like this to occur." Among the new rules: requiring two medical professionals to be involved when the drug is administered, she said.

According to the criminal complaint:

For more than three months, from November 2010 to March 2011, Zenner sneaked into drug lockboxes at the hospital and withdrew the painkiller from intravenous bags intended for patients.

To conceal some of those thefts, Zenner refilled the bags with saline and returned them to the lockboxes.

Zenner has been charged by information, indicating that he admits to the allegations against him.

His attorney, Paul Engh, said that Zenner has completed addiction treatment, has accepted responsibility for the thefts and has "a deep and abiding regret" for his actions.

Sentencing is scheduled for Sept. 26, and Engh said his client is hoping for a "merciful result."

Zenner has been a licensed nurse in Minnesota for more than 20 years, most recently as a registered nurse. He surrendered his license in March 2011, soon after the Minnesota Board of Nursing was notified of the allegations.

He was hired as a nurse at St. Cloud Hospital in March 1994, Nistler said.

Zenner graduated from St. Cloud Technical College in 1992, according to the nursing board and also received a degree in 1998 from North Hennepin Community College.

Paul Walsh • 612-673-4482